Offshore evidence of large–scale lateral collapse on the eastern flank of Stromboli, Italy, due to structurally–controlled, bi-lateral flank instability

New detailed swath bathymetry and long-range side scan sonar data on the submerged flanks of Stromboli, integrated with seismic data and seabed sampling, indicate that repeated lateral instability processes occurred on the eastern flank of the volcano, although no debris avalanche deposits were known before the high-resolution exploration of the seabed. This flank of the island is opposite to the northwestern side, more evidently affected by repeated flank collapses, and this setting is evidence for a structurally-controlled, bilateral flank instability of the volcanic edifice. Evidence of at least two large-scale lateral collapses is represented by a superficial hummocky megablock field, partially outcropping on the middle-lower eastern submerged slope, and by a chaotic unit (estimated volume is 1-2 km(3)) actually embedded within the volcaniclastic sequence at the foot of the submerged flank. A main submarine scar open towards the SE is carved in the upper slope on the eastern flank of the island, being partially filled with gravity flow deposits owing to the present-day depositional setting of the volcano's submerged flank. A morphological continuity can be envisaged between this submerged scar and an inferred subaerial one, likely encompassing the Rina Grande depression and several minor nested landslide scars recognized on the eastern Stromboli flank. A spatial and temporal reconstruction of possible events is proposed. The chaotic debris avalanche unit, buried within the volcaniclastic apron at the slope base and partially reworked in its distall part within the Stromboli Canyon floor, is thought to be the result of a lateral collapse event (called Paleostromboli Lateral Collapse, PLC, as it likely occurred in the earlier stages of the subaerial development of the island) affecting a wide sector of the eastern Stromboli flank. The megablock field might instead result from a more superficial debris avalanche deposit, partially eroded by gravity flows currently acting on the seabed. It is tentatively related to the recent subaerial scar morphologies affecting the Vancori units (and for this reason named Vancori Lateral Collapse, VLC). Displaced materials have lower volume and mobility than the PLC. (C) 2009 Elsevier B.V. All rights reserved.